Photo-thermal conversion characteristics of MWCNT-H2O nanofluids for direct solar thermal energy absorption applications

Stable water-based multi-walled carbon nanotube (MWCNT) nanofluids with mass fractions ranging from 0.0015% to 0.1% were prepared. To investigate the photo-absorption properties and photo-thermal conversion performance of MWCNT-H2O nanofluids, heating and light-irradiation cycling tests were experimentally performed at temperatures below 90 °C. Heat treatment is beneficial to improve the optical absorption capability of MWCNT-H2O nanofluids, and there existed an optimal concentration of about 0.01 wt% with respect to the spectral transmittance and extinction coefficient. Extinction coefficients of MWCNT-H2O nanofluids increased linearly with the MWCNT concentration in a range of 0-0.01 wt% at wavelengths of 500-900 nm. Compared to DI water, the temperature of MWCNT-H2O nanofluid at the optimal mass fraction of 0.01% was increased by about 14.8 °C (or 22.7%) after a light irradiation time of 45 min. The photo-thermal conversion performance was enhanced with increasing the light irradiation cycle at appropriate concentrations of nanofluid, and the probable mechanism caused by the CNT-fiber agglomeration was qualitatively analyzed. The receiver efficiency decreased with increasing the light irradiation time, and the highest value was up to 96.4% at 0.01% mass fraction. The prospects for possible applications of MWCNT-H2O nanofluids in low-temperature direct solar thermal energy absorption were presented.